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I don't know why threading._get_ident() (which is _thread.get_ident()) is private: this function is safe.
This function becomes useful with pthread_kill() which will be added by #8407.

I think that threading._get_ident() is more reliable than threading.current_thread().ident because Thread.ident can be None in some cases.

@pitrou: What do you think Antoine?

The threading API changed in Python 3 with the PEP 3108, especially and issue #2875. Extract of the PEP:
"thread [done]
 * People should use 'threading' instead.
 * Rename 'thread' to _thread.
 * Deprecate dummy_thread and rename _dummy_thread.
 * Move thread.get_ident over to threading.
 * Guido has previously supported the deprecation [13]."
The issue was closed by the commit [7fe3a8648ce2], "Issue #2875: Rename thread to _thread and dummy_thread to _dummy_thread."

> I think that threading._get_ident() is more reliable than
> threading.current_thread().ident because Thread.ident can be None in
> some cases.
In which case can it be None?

> In which case can it be None?
Oh, I misunderstood threading.py. current_thread().ident cannot be None.
During the bootstrap of a thread, Thread._ident is None, but current_thread().ident is not None because current_thread() creates a dummy thread object having the right identifer. This dummy object is destroyed at soon as the Thread object is started (see Thread._bootstrap_inner).
current_thread().ident is a little bit "suboptimal" because it gets the identifier of the thread, it reads the corresponding thread object, and then it gets the identifier of the thread object.
def current_thread():
 ...
 return _active[_get_ident()]
 ...

I find it strange that you propose to make a function public and remove its doc :) Is a negative missing in the bug title?

threading_get_ident.patch: make get_ident() public, replace threading._get_ident() by threading.get_ident().
According to this patch, get_ident() function *is* used: it is used by the logging and reprlib modules (and many tests). My patch avoids the usage of the low-level module, _thread_, in logging and reprlib.
I adapted signal.pthread_kill() documentation to replace threading.current_thread().ident by threading.get_ident().

New changeset 5548197d1f22 by Victor Stinner in branch 'default':
Close #12028: Make threading._get_ident() public, rename it to
http://hg.python.org/cpython/rev/5548197d1f22 

New changeset 6387aa6b496d by Victor Stinner in branch 'default':
Issue #12028: fix NEWS entry
http://hg.python.org/cpython/rev/6387aa6b496d